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7<title>The MPL Reference Manual: if_</title>
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11<table class="header"><tr class="header"><td class="header-group navigation-bar"><span class="navigation-group"><a href="./type-selection.html" class="navigation-link">Prev</a>&nbsp;<a href="./if-c.html" class="navigation-link">Next</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group">Back&nbsp;<a href="./if-c.html" class="navigation-link">Along</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./type-selection.html" class="navigation-link">Up</a>&nbsp;<a href="../refmanual.html" class="navigation-link">Home</a></span><span class="navigation-group-separator">&nbsp;|&nbsp;</span><span class="navigation-group"><a href="./refmanual_toc.html" class="navigation-link">Full TOC</a></span></td>
12<td class="header-group page-location"><a href="../refmanual.html" class="navigation-link">Front Page</a> / <a href="./metafunctions.html" class="navigation-link">Metafunctions</a> / <a href="./type-selection.html" class="navigation-link">Type Selection</a> / <a href="./if.html" class="navigation-link">if_</a></td>
13</tr></table><div class="header-separator"></div>
14<div class="section" id="if">
15<h1><a class="toc-backref" href="./type-selection.html#id446" name="if">if_</a></h1>
16<div class="section" id="if-synopsis">
17<h3><a class="subsection-title" href="#synopsis" name="synopsis">Synopsis</a></h3>
18<pre class="literal-block">
19template&lt; 
20      typename C
21    , typename T1
22    , typename T2
23    &gt;
24struct <a href="./if.html" class="identifier">if_</a>
25{
26    typedef <em>unspecified</em> type;
27};
28</pre>
29</div>
30<div class="section" id="if-description">
31<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
32<p>Returns one of its two arguments, <tt class="literal"><span class="pre">T1</span></tt> or <tt class="literal"><span class="pre">T2</span></tt>, depending on the value <tt class="literal"><span class="pre">C</span></tt>.</p>
33</div>
34<div class="section" id="if-header">
35<h3><a class="subsection-title" href="#header" name="header">Header</a></h3>
36<pre class="literal-block">
37#include &lt;<a href="../../../../boost/mpl/if.hpp" class="header">boost/mpl/if.hpp</a>&gt;
38</pre>
39</div>
40<div class="section" id="if-parameters">
41<h3><a class="subsection-title" href="#parameters" name="parameters">Parameters</a></h3>
42<table border="1" class="table">
43<colgroup>
44<col width="15%" />
45<col width="36%" />
46<col width="48%" />
47</colgroup>
48<thead valign="bottom">
49<tr><th>Parameter</th>
50<th>Requirement</th>
51<th>Description</th>
52</tr>
53</thead>
54<tbody valign="top">
55<tr><td><tt class="literal"><span class="pre">C</span></tt></td>
56<td><a class="reference" href="./integral-constant.html">Integral Constant</a></td>
57<td>A selection condition.</td>
58</tr>
59<tr><td><tt class="literal"><span class="pre">T1</span></tt>, <tt class="literal"><span class="pre">T2</span></tt></td>
60<td>Any type</td>
61<td>Types to select from.</td>
62</tr>
63</tbody>
64</table>
65</div>
66<div class="section" id="if-expression-semantics">
67<h3><a class="subsection-title" href="#expression-semantics" name="expression-semantics">Expression semantics</a></h3>
68<p>For any <a class="reference" href="./integral-constant.html">Integral Constant</a> <tt class="literal"><span class="pre">c</span></tt> and arbitrary types <tt class="literal"><span class="pre">t1</span></tt>, <tt class="literal"><span class="pre">t2</span></tt>:</p>
69<pre class="literal-block">
70typedef <a href="./if.html" class="identifier">if_</a>&lt;c,t1,t2&gt;::type t;
71</pre>
72<table class="field-list" frame="void" rules="none">
73<col class="field-name" />
74<col class="field-body" />
75<tbody valign="top">
76<tr class="field"><th class="field-name">Return type:</th><td class="field-body">Any type.</td>
77</tr>
78<tr class="field"><th class="field-name">Semantics:</th><td class="field-body">If <tt class="literal"><span class="pre">c::value</span> <span class="pre">==</span> <span class="pre">true</span></tt>, <tt class="literal"><span class="pre">t</span></tt> is identical to <tt class="literal"><span class="pre">t1</span></tt>; otherwise <tt class="literal"><span class="pre">t</span></tt> is
79identical to <tt class="literal"><span class="pre">t2</span></tt>.</td>
80</tr>
81</tbody>
82</table>
83</div>
84<div class="section" id="if-example">
85<h3><a class="subsection-title" href="#example" name="example">Example</a></h3>
86<pre class="literal-block">
87typedef <a href="./if.html" class="identifier">if_</a>&lt;true_,char,long&gt;::type t1;
88typedef <a href="./if.html" class="identifier">if_</a>&lt;false_,char,long&gt;::type t2;
89
90<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t1, char&gt; ));
91<a href="./assert.html" class="identifier">BOOST_MPL_ASSERT</a>(( is_same&lt;t2, long&gt; ));
92</pre>
93</div>
94<div class="section" id="if-see-also">
95<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
96<p><a class="reference" href="./metafunctions.html">Metafunctions</a>, <a class="reference" href="./integral-constant.html">Integral Constant</a>, <a class="refentry reference" href="./if-c.html"><tt class="refentry literal"><span class="pre">if_c</span></tt></a>, <a class="refentry reference" href="./eval-if.html"><tt class="refentry literal"><span class="pre">eval_if</span></tt></a></p>
97<!-- modtime: November 10, 2004 04:40:27 +0000 -->
98<!-- Metafunctions/Type Selection//if_c |20 -->
99</div>
100</div>
101
102<div class="footer-separator"></div>
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